default track value set to true in MKVFile.command() even when value is False in the object

[sjb-git]

Describe the bug
When an mkv mux command is created from an mkv object: the default track value is ignored, pymkv only checks whether the default_track value exists. So if the value exists and is false, the value is set to true in the mux command. This may be the intended behaviour but it seems counterintuitive to me. This occurs because track.default_track is a string value, not a boolean value.

To Reproduce
Here is the code from line 168 of the current version:
if track.default_track:
command.extend(['--default-track', str(track.track_id) + ':1'])
else:
command.extend(['--default-track', str(track.track_id) + ':0'])
Clearly it checks for the existence of the value but does not replicate the value in the command.

Expected behavior
That a value of 0 or False would: command.extend(['--default-track', str(track.track_id) + ':0'])

Screenshots
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Software (please complete the following information):

• OS: [e.g. macOS 10.14]
• MKVToolNix version [e.g. v44.0.0]

Additional context
Add any other context about the problem here.